This invention relates to a process for stabilizing acicular ferromagnetic iron or iron-alloy particles, especially, having an averaged particle-size smaller than 1.mu. against the oxidation thereof as contacted with the air. More particularly, this invention relates to the production of novel acicular iron or iron-alloy fine particles having stable magnetic properties in the air at a temperature below 100.degree. C.
In recent years, a demand for a high efficiency of magnetic recording media has more and more increased with the progress in miniaturizing and lightening a magnetic recording and reproducing apparatus. Namely, improvements in a high density recording and a high output characteristic, in particular, a high frequency characteristic have been demanded in the recording media. Magnetic material is therefore required to have a large saturated magnetic flux density and a high coercive force to satisfy the above demand.
Magnetic materials conventionally employed so far as magnetic recording media include magnetic powder such as magnetite, maghemite and chromium dioxide, which have saturated magnetic flux density, .sigma..sub.s, between about 70-85 emu/g and coercive force, H.sub.c, between 250-500 O.sub.e.
Referring, in particular, to the oxide magnetic powder, it has saturated magnetic flux density at most about 85 emu/g and generally between about 70-80 emu/g, which forms a main factor to restrict the limits for the reproducing outputs and the density in recording. Further, while Co-containing magnetic powder, that is, Co-Magnetite or Co-maghemite magnetic powder has also been used and characterized by its coercive force as high as 400-800 O.sub.e, the saturated magnetic flux density thereof is only as low as 60-80 emu/g.
Meanwhile, it is well known in the art that magnetic powder of acicular iron or iron-alloy particles which is substantially pure iron or an iron-alloy shows more excellent magnetic properties, that is, a greater saturated magnetic flux density, .sigma..sub.s, (for example, between 90-200 emu/g) and a higher coercive force, H.sub.c, (for example, between 600-2000 O.sub.e) as compared with the foregoing conventional oxide type magnetic powder.
As far as the inventors are informed, however, those acicular iron or iron-alloy particles prepared by the reduction of acicular iron oxide have not yet been used on an industrial scale as magnetic recording material because of their significant unstability to oxidation.
Since these acicular iron or iron-alloy particles are extremely fine, being less than 1.mu., and the activity on the particle surfaces is very high, a violent oxidation together with heat development results upon exposing them in the air.
Such a significant unstability of the acicular iron or iron-alloy particles, is pointed out, for example, as follows: Japanese patent application laying open No. 135835/1974 discloses that even a magnetic alloy powder which has become noted as a high density magnetic recording material in place of the conventional iron oxide magnetic powder still shows such an unstability as resulting in a violent oxidation to fire when taken out as it is in the air just after the production, because the powder is finely divided into a size as small as several .mu.. Japanese patent application laying open No. 97738/1974 also discloses that acicular iron or iron-alloy particles have not yet been put to practical use at present. One of the causes is a problem of oxidation-resistant property of the metallic magnetic powder in the air. The metallic magnetic powder, if left in the air, gradually loses its magnetic property with the progress of oxidation and, in the worst case, ignites spontaneously to burn in a moment due to external heating, mechanical shock, electrostatic discharging or the like. In the fine particles such as those used for the magnetic powder whose size is greatly decreased, the contaction area to the air generally increases due to the increase in the surface area per unit weight (specific surface area) and the reactivity thereof is no more negligible. Therefore, the metallic magnetic powder produced through reduction, if taken out from the furnace as it is and contacted to the air, spontaneously ignites to burn out because of the violent oxidation.
In view of the above, several processes for stabilizing acicular iron or iron-alloy particles against the oxidation have already been proposed in the prior art as summarized below.
Japanese patent application laying open Nos. 112465/1976, 97738/1974, 135835/1974, 12958/1972, 5057/1971 and the like disclose methods comprising treating native acicular iron or iron-alloy particles with organic solvents. In the above methods, the organic solvent coating the acicular particles is gradually evaporated in the air to result in a moderate contaction between the particles and the air thereby forming a thin oxide film on the surface of the particles.
Co-existence of the igniting acicular fine metallic particles and the combustible organic solvent is, of course, very dangerous and since it imposes a heavy burden on the production control cost, the methods are not suited to industrial practice. Moreover, the above methods give much troubles to the control of the evaporating state of the organic solvent and hence a difficulty in the control of the formation of metal oxide coating film.
Meanwhile, Japanese patent application laying open No. 79153/1973 discloses a method comprising the steps of moderatedly oxidizing the surface of the native acicular iron particles over a long time with a gaseous mixture consisting of 1% air and 99% N.sub.2. Unfortunately, this method results in much trouble in the control of the oxidizing degree at the surface, requires a long time for the treatment and hence is not suited to the industrial practice.